The invention relates to spheroidal sintered particles in the grain size range from 20 to 180 xcexcm and to a process for producing these particles.
In the prior art, sintered particles are used in combination with further hard materials and metallic additives, using various methods, to apply the sintered particles to a substrate which is to be protected against wear and to produce infiltration components with a particular resistance to wear. The sintered particles consist of a hard metal containing sinterable hard material particles, e.g. tungsten carbide grains. The tungsten carbides which are used in the prior art primarily as wear-resistant components are usually embedded in a metallic matrix. Typical examples of the matrix material are the metals cobalt, chromium, nickel, copper and iron or mixtures or alloys thereof.
One particular field of application for sintered hard metal particles is the reinforcing of rock bits used in drilling for oil. The prior art which is relevant to this application area derives, as far as the Applicants are aware, from U.S. Pat. No. 5,791,422, US RE 37,127, US 4,944,774 and US 5,944,127.
U.S. Pat. No. 5,791,422 describes a drill, the teeth of which have a hard facing which contains from 20 to 50% by weight of steel and from 50 to 80% by weight of a filler, the filler including 10 to 100% by weight of particles which consist of fused tungsten carbide and have a particle size of between approximately 16 and 40 mesh (390-1000 xcexcm).
Fused tungsten carbide is typically a eutectic mixture of tungsten and carbon, where manufacturing conditions lead to a phase mixture comprising a WC phase and a W2C phase is established. It is therefore substoichiometrically carburized, i.e. contains less carbon than the more desirable WC phase.
U.S. Pat. No. RE 37,127 describes a wear-resistant hard material composition which contains at least 60% by weight of granules which contain a fraction of sintered spheroidal cermets and a fraction of pellets of fused tungsten carbide. A range between approximately 16 and approximately 30 mesh (500-1000 xcexcm) is specified for the grain size of the sintered carbide pellets.
U.S. Pat. No. 4,944,774 describes a hard facing composition for teeth of a rock bit, the composition containing a monocrystalline WC and a sintered and fragmented tungsten carbide cermet, the grain sizes being greater than 20 mesh (780 xcexcm).
Moreover, U.S. Pat. No. 5,944,127 discloses a hard facing material for rock bits in which there are crystalline WC particles which have a particle size of less than 200 mesh (75 xcexcm). It is stipulated that it is preferable for the grain size range of the particles to be between 30 and 70 xcexcm.
Apart from U.S. Pat. No. 5,944,127, all the compositions described for sintered powder have the drawback that the grain size of the particles which protect against wear is of an order of magnitude which is unsuitable for certain applications. For example, sand which occurs in the ground has a particle size which may be considerably smaller than the grain size of the carbides used. The inventors have recognized that protection against wear from these particles is more effective if the size of the hard material particles is in the same order of magnitude as the attacking particles.
The hard material composition described in U.S. Pat. No. 5,944,127 contains carbides with a grain size of the same order of magnitude as very fine abrasive particles which are found in the ground. However, the sintered material described has the drawback that, although the tungsten carbides used have a very high hardness, the impact strength and ductility are not optimum, on account of their materials properties.
As a further application area for sintered hard materials, U.S. Pat. No. 5,733,649, US 5,733,664 and US 5,589,268 describe the production of hard material mixtures for the production of infiltrated components, such as diamond drill bits for oil drilling, as part of the prior art. In these patents, the composition of the hard material mixtures, which are referred to as matrix powders, comprises sintered and fragmented cermets of the tungsten carbide/cobalt type, containing from 5 to 20% by weight of metallic binder and having a grain size range of 38 to 125 xcexcm. In addition, monocrystalline tungsten carbide hard materials with a grain size of 45 to 180 xcexcm and fused tungsten carbides with a grain size of less than 53 xcexcm are also used. The grain size distributions, which are adapted to one another, are intended to give the infiltration component which is subsequently produced therefrom a good resistance to erosion in addition to a good resistance to abrasion.
Proceeding from here, the invention is based on the object of providing particles which, applied to a substrate or processed to form a compact body by infiltration, offer improved wear properties with regard to washing out of the matrix and impact strength. Furthermore, it is intended to describe a process for producing a material of this type.
This object is solved by providing sintered hard metal particles of substantially spheroidal shape having a grain size of 20 to 180 xcexcm. These particles have a predominately closed porosity or are substantially free of pores. The decisive advantage of the new, densely sintered pulverulent sintered material is based on the combination of a very high hardness and impact strength with a favourable outer shape with regard to attack from abrasive particles. Especially the small grain size of the particles which is in the same order of magnitude as attacking particles, makes the material eminently suitable for use in reinforcements preventing mineral wear. By way of example, in a process end product consisting of 94% by weight of tungsten carbide and 6% by weight of cobalt, there may be particles which have a hardness of 1380-1690 HV 0.3, the grain size range being 63-106 xcexcm. Furthermore, the closed structure of these particles prevents the penetration of molten metal during processing, so that there are no dissolution phenomena in the core.
Such particles can be produced by a process, comprising the successive steps of:
a) providing a substantially spheroidal powder starting material
b) introducing the powder starting material into a furnace,
c) sintering the powder starting material by heating to a temperature at which the material of the metallic binder adopts a pasty state, and then by applying a gas pressure to reduce the pore content of the starting material, and
d) cooling the spheroidal sintered particles and removing the spheroidal sintered particles from the furnace.